The present invention is related to a process for inhibiting corrosion of metal surfaces which are in contact with H.sub.2 S-containing acid gas streams treated with an aqueous amine-containing scrubbing solution. Corrosion is inhibited by use of a thiosulfate salt in the presence of sulfide or hydrosulfide ions.
The sweetening of natural and synthetic gases has been practiced for many years. Typically, this has involved the removal of acidic compounds such as CO.sub.2, H.sub.2 S and COS by absorption of the acid gases in a liquid absorbent medium. Various aqueous, absorbing or scrubbing solutions have been utilized to remove the acidic components. Such solutions include those containing alkanolamines, sulfolane (2, 3, 4, 5-tetrahydro-thiophene-1, 1-dioxide); 2, 3, 4, 5-tetrahydrothiophene-1, 1-dioxide with diisopropanol amine and potassium carbonate solutions. Each of these systems experiences corrosion of metal surfaces in contact with the scrubbing solution, which may be attributable to one or more of the following: decomposition of the scrubbing solution; reaction of the acidic components of the gas and the absorbent; and direct attack by the acidic components in the gases. This corrosion may occur throughout the entire gas treating system on metal surfaces in contact with the solutions and the acid gas.
U.S. Pat. No. 3,887,330 discloses a method for preventing sulfur corrosion of ferrous materials in pipelines wherein ammonium polysulfide as an aqueous solution is mixed with an essentially non-aqueous sulfur hydrocarbon slurry. The ammonium polysulfide may be externally added or may be formed by reaction of ammonium hydrosulfide with free sulfur in the slurry.
U.S. Pat. No. 3,438,680 describes a similar method for transporting sulfur particles through a pipeline wherein sulfur particles are intimately mixed with an aqueous solution of a polysulfide salt to form a slurry, the slurry is pumped through a pipeline and the sulfur particles are removed from the aqueous solution. The polysulfide salt used as carrier is reported to prevent corrosion.
Use of alkali metal sulfides per se as corrosion inhibitors in ammonium nitrate-urea deicer compositions is reported in U.S. Pat. No. 3,297,577.
For some gas treating processes, i.e., those characterized as hot carbonate processes, it is reported that hydrogen sulfide gas can inhibit corrosion. See Bienstock et al., Corrosion, 17, 571 (1961), Benson et al., Petroleum Refiner, 39, 127 (1960) and a pamphlet from the U.S. Department of the Interior Bureau of Mines, "Removing Hydrogen Sulfide By Hot Potassium Carbonate Absorption", by Field et al. (1954). However, for most gas treating processes using aqueous amine solutions, H.sub.2 S per se is not an effective corrosion inhibitor. Process designs of this type are well known to limit the acid gas content of such solutions in order to minimize corrosion.